The invention relates to a method for avoiding incorrect combustion misfire fault detection in a motor vehicle with a dual mass flywheel.
Legal regulations (for example, in the USA) require the internal combustion engine of a motor vehicle to be monitored for combustion misfires. Since a combustion misfire or a main exciting ignition order contributes to a low frequency order at the crank shaft encoder wheel, prior art detection methods attempt to determine low frequency oscillations at the crank shaft encoder wheel of the engine. This is currently carried out by evaluating the crank shaft signal. In this context, the times at which defined angular segments of the current cylinder pass through are compared with the times when other cylinders pass through. If the segment time is exceeded repeatedly, an ignition misfire or combustion misfire is determined and a fault entry is made in the control unit.
An example of such a detection method is disclosed in DE 10 2011 085 212 A1. In this method for avoiding incorrect combustion misfire fault detection, initially, in order to detect (individual) combustion misfires, a characteristic variable which is dependent on the acceleration of the internal combustion engine is determined continuously by a first monitoring method during the ongoing operation of the internal combustion engine and is compared with a predefined variable threshold value. If the corresponding threshold value is exceeded by the determined characteristic variable, a combustion misfire is said to be detected. The characteristic variable (irregular running) can be, for example, the time when defined angular segments pass through.
Since the low frequency oscillations of the crank shaft encoder wheel, which are detected by the known method, can occur not only in the case of ignition misfires and combustion misfires, but also as a result of operating time specific drive train oscillations, the operating point specific oscillations of the drive train should not be taken into account or should not be detected as combustion misfires. For example, in what is referred to as the float operating point at which a torque about the zero load is generated (i.e. the driver requests an engine torque which is equal to or slightly higher than the engine's own drag torque), the low frequency oscillations by the drive train can reach the same order of magnitude as the oscillations caused by combustion misfires.
With the known method it is possible to differentiate fluctuations in the rotational speed which are caused by combustion misfires from fluctuations in the rotational speed which result from an operationally induced drive train oscillation. This occurs, here, in such a way that pre-conditioning of the drive train is detected in the engine controller by way of a second monitoring method, during which pre-conditioning low frequency oscillations can occur at critical motor vehicle operating points owing to drive train oscillations. During detection of such pre-conditioning, an increase in the threshold value is subsequently carried out, with the result that these operationally induced oscillations of the drive train are not incorrectly detected as combustion misfires. Therefore, if a driving maneuver with corresponding pre-conditioning is detected, the threshold value for the misfire detection for this critical operating point of the internal combustion engine is corrected upward in order to avoid the incorrect detection of misfires. If the critical operating points are passed through without pre-conditioning, the original threshold value is retained and the threshold value is not adapted. The range in which the pre-conditioning or the critical excitation takes place is dependent on the torque range and rotational speed range and can be determined within the scope of simulations or during vehicle trials. The level of the raising of the threshold value can be determined, for example, in advance by way of measurements and stored in a characteristic diagram.
Such pre-conditioning during which an increase in the threshold value is carried out, occurs, for example, when the bow springs which are arranged in a dual mass flywheel are prestressed. If the bow springs are prestressed and if, at the same time, a high rotational speed is present, it is not possible to relieve the stress of the bow springs. As a result, in the case of low torques bounce processes may occur in the dual mass flywheel, which in turn leads to oscillations at the crank shaft. If the threshold value were to be retained, combustion misfires would be incorrectly detected. Therefore, incorrect detection of combustion misfires can be reduced in such critical driving situations by increasing the threshold value.
Finally, the threshold value is reduced to the normal value or the increase is advantageously canceled out if the critical motor vehicle operating point is exited again and/or if the pre-conditioning is ended.
The object of the invention is to provide a method for avoiding incorrect combustion misfire fault detection which is improved compared to the prior art.
This and other objects are achieved according to the invention by providing a method for avoiding incorrect combustion misfire fault detection in an internal combustion engine in a motor vehicle with a dual mass flywheel. In order to detect combustion misfires, a characteristic variable (irregular running), which is dependent on the acceleration of the internal combustion engine, is determined continuously during the ongoing operation of the internal combustion engine and compared with a predefined irregular running threshold value. When the irregular running threshold value is exceeded, a combustion misfire is detected, wherein the frequency of detected combustion misfires (for example, at a specific number of crank shaft revolutions) is detected, and when a defined frequency threshold is exceeded, a combustion misfire fault detection is activated. According to the invention, basically when a dual mass flywheel bounce is detected with the simultaneous detection of combustion misfires, the injection of at least one cylinder (but preferably all the cylinders) is reduced for a predefined frequency or for a predefined time (at least once). The reduction of the injection can also be a complete switching off of the injection.
The invention is based on the following.
So called dual mass flywheel (DMF) bounce is a problem which has been known in the area of internal combustion engines since the introduction of the dual mass flywheel. A DMF is used for acoustic reasons. For example, hard engagement after a reverse gear shift operation or during a clutch engagement process in the same gear can cause the bow spring in the DMF to be tightened. If the engine is then operated with a positive load, DMF bounce can occur in specific characteristic rotational speed load ranges. The bounce brings about increased irregular running, which can be misinterpreted as combustion misfires by the sensor system. This incorrect detection of combustion misfires can result in cylinder shut off and the starting of an emergency running program (for example, with torque reduction).
A mechanical solution to this problem would be possible, but is unfavorable for reasons of cost and space.
The bouncing of the dual mass flywheel can already be detected by known methods. According to the invention, basically after a DMF bounce has been detected with the simultaneous detection of (individual) combustion misfires, an injection reduction is carried out, preferably injection shut off. The (known) “combustion misfire fault detection” waits for a reaction (for example, fault entry, cylinder shut off, etc) for a predefined frequency of individual combustion misfires (instances of the threshold value being exceeded). The term “combustion misfire fault detection” is therefore understood to mean the exceeding of the threshold value in respect of the frequency of individual combustion misfires (frequency threshold value). The combustion misfire fault detection is therefore to be differentiated from the detection of (individual) combustion misfires. The injection reduction according to the invention is preferably carried out after a predefined pre-threshold value relating to the frequency of the detection of combustion misfires is reached, but before the “combustion misfire fault detection”. The detection of a combustion misfire is usually carried out by the known irregular running measurement by way of the sensor system in order to measure the engine speed.
The injection reduction results in a negative load surge (=torque surge) which relieves the bow springs of the DMF of stress. The injection is preferably reduced or suppressed at least once at all the cylinders so that the negative load surge is as strong as possible and therefore reliably performs its purpose. In order to increase the load surge, a brief increase in the torque can be carried out before the injection reduction in one development of the invention.
The invention provides a cost effective and reliable method for avoiding incorrectly detected combustion misfires since the bounce itself is therefore ended before the combustion misfire fault detection responds.
This method provides the following advantages. A significant advantage is the fact that the implementation of the function does not give rise to any significant costs since no additional hardware components are necessary. Instead, an internal damper, which would otherwise be necessary in the dual mass flywheel (and which is intended to reduce the oscillations) can even be dispensed with. As a result, there can in turn be a savings both in terms of cost and in weight.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.